The invention resides in the field of quantitative determination of volatile compounds in liquids. In particular, the invention relates to an improved gas permeable tubing probe for measuring one or more volatile compounds in liquids or fluids.
Gas sensors for determining the concentration of volatile compounds in liquids are in wide use in a variety of environments, e.g., chemical plants, solution agro-food plants, beverage industry, fermentation broths, etc. For example, the alcohol industry produces ethanol by fermentation while the biotechnology community uses methanol-oxidizing micro-organisms to produce either valuable research reagents or clinical/commercial products. Therefore, there is an increasing need for measuring, in real-time, methanol concentration or that of any other volatile of interest in fermentation broths or in any liquids in order to control that concentration and to prevent or minimize hazards. There are currently a few approaches, which are commonly used, to analyse volatile compounds in fermentation broth. One is to analyse exhausted gases in the non-sterile section of the fermenter and another one is to analyse the volatile(s) through the use of a carrier gas and of a permeable membrane or tubing made of silicone or PTFE (Teflon).
The first approach gives a very slow response regarding the concentration of the particular volatile and is sensitive to changes in the airflow rate, which makes control very difficult. This is often the case when the dissolved oxygen level is controlled by varying the aeration rate.
In the second approach, a probe made of a gas permeable tube is inserted into the fermentation broth. A carrier gas passing through the tube collects volatiles that pervaporate through the tube from the broth. A volatile detector is connected at the outlet of the tube to measure the concentration of the volatile(s) of interest. U.S. Pat. No. 5,204,262, Apr. 20, 1993, Meiering et al, describes an ethanol sensor for computerized fermentation control. It makes use of a rigid Teflon tubing mechanically tied between the inlet and outlet gas carrier ports. The ports are part of an empty rigid tube responsible for bringing the small carrier gas tube to the ports. In the patent, the probe consisting of a piece of tube is inserted from the top of the fermenter. This design, however, limits the use of the probe to small-scale fermenters and does contain a sealing mechanism to ensure sterility. None of the tubing probes presently available is compatible with the 25 mm port used with most fermenters.
U.S. Pat. No. 4,404,284, Sep. 13, 1993, Heider et al, presents a technique for measuring volatile constituents of a culture medium, in that a permeable membrane and a carrier gas are used. In the patent, the probe consists of a silicone membrane slid on to a cylindrical core whose exterior is provided with helical ducts. The silicone membrane and the helical ducts form a helical channel through which a carrier gas and volatiles are transported. The carrier gas containing the volatile(s) of interest is brought back to a sensor either inside or outside the body of the probe. The probe is made of two parts machined with precision that force the carrier gas in and out of the body. This probe works well and is compatible with 25 mm ports. The probe is, however, more complicated to build, more expensive and the membrane is very fragile. This constitutes a real problem whenever membrane perforation occurs and the contents of the fermenter, especially at the larger scales, must be thrown away causing important losses of money and time.
There is, therefore, a need for a sterilisable tubing probe that can be inserted in any port, including the 25 mm side port of common fermenters (bioreactors). The probe must be robust and easy to build, and must permit its use whatever the fermenter size, including pilot plant fermenters (20L to 1500L, or more).
In accordance with one aspect, the invention provides a volatile extraction probe designed in such a way that it can be inserted into a fermenter. The probe has a body made of a single piece of metal joined with a perforated plate supporting a piece of gas permeable tube of varying length for transporting a carrier gas and the volatile(s) of interest from the fermenter to the outside.
In accordance with another aspect, the probe of the invention is designed so that substantially only the gas permeable tube is located inside the fermenter.
In accordance with still another aspect, the probe is operable with a variety of lengths of a gas permeable tube so that the probe can be optimized to suit volatile(s) of interest and operating conditions.
In accordance with still another aspect, the tube is threaded in and out of a supporting plate avoiding, therefore, any mechanical stress to the tube due to agitation or to aeration. This permits operation with a variety of lengths of a gas permeable tube so that the probe can be optimized to suit volatile(s) of interest and operating conditions.
In accordance with yet another aspect, the probe of the invention is easy and economical to manufacture due to a single body construction. It is also easy to maintain because it makes use of readily available silicone tubing of any length and diameter.
In accordance with yet another aspect, the probe of the invention is steriliable-in-place due to its sealing mechanism and easy but solid tubing attachment. The design of the body is made to fit in a standard 25 mm port but by redesigning the body section it can be easily adapted to other commonly found port (flanged, tri-clamp, threaded) in industry.
In accordance with a further aspect, the invention is directed to a tubing probe for measuring a volatile compound in a fluid contained in an enclosure. The probe comprises a probe body made of a single piece of metal and inlet and outlet channels provided in the probe body. The probe further includes a gas permeable tube attached to one end of the probe body to form a continuous passage for a carrier gas between the inlet and outlet channels, and a sealing mechanism between the enclosure and the probe body for positioning substantially only the gas permeable tube inside the enclosure so that when the probe is inserted into the enclosure, the volatile compound pervaporates into the gas permeable tube.
In accordance with a further aspect, the outlet of the probe, having a carrier gas enriched with the volatile compound(s), directs the carrier gas+volatiles mixture to the top center of the body where a sensor headset can be screwed on.